1. Field of the Invention
This invention relates generally to a system for correlating electrical signals and, more particularly, to an optical system for simultaneously performing both in-phase and quadrature phase correlation of wide bandwidth electrical signals using a multi-mode imaging device and a predetermined modulation format.
2. Discussion of the Related Art
The need to correlate electrical signals is an important operation for certain signal processing systems. The correlation of electrical signals is a mathematical procedure that causes two or more input signals to be aligned and then compared to provide increased signal processing gain and sensitivity. By aligning the signals in time, and then multiplying (mixing) and integrating the signals, the amplitude of the combined signals will significantly increase. This allows weaker signals to effectively be separated from receiver noise. By identifying peaks in signal intensity from the correlation process, a delay between the signals being correlated can be identified to align the signals. This gives the delay between signals from a common source that are received by receivers separated in space. Examples of systems that require correlation of electrical signals include passive millimeterwave (PMMW) imaging systems, nulling antenna signal processing systems, and RADAR return signal and processing systems. Matched filter correlation and processing of wideband pulsed or spread spectrum signals can be useful for pulsed or digital communications.
State of the art correlation systems include analog and digital systems that provide point-to-point analysis of two or more electrical signals. For an analog correlation system, the input signals are multiplied (mixed) on a time-by-time basis and the product is integrated. For a digital correlation system, the input signals are multiplied on a sample-by-sample basis and then summed to provide the correlation. Optical systems are also known that correlate radio frequency or electrical signals. Conventional opto-electronic correlators typically employ acousto-optical Bragg cells that receive an optical reference beam and the RF signal to be correlated. Additional optical processing elements are used to provide the actual multiplication and integration of the optical signals to provide the correlation. The article N. Ross Price et al., xe2x80x9cLinear electro-optic effect applied to a radio astronomy correlator,xe2x80x9d Radio Science, V. 31, No. 2, March-April 1966, pgs. 451-458 discloses a known opto-electronic correlator.
The known correlation processing systems have heretofore been acceptable for providing correlation of two or more electrical signals within a relatively narrow bandwidth that does not have an appreciable spectral content. Such known correlation systems have not been able to provide efficient correlation above about 1 GHz, and thus their performance is limited for providing correlation of relatively wideband electrical signals. For conventional RF analog correlators, the correlation bandwidth is limited by the power divider, 90xc2x0 hybrid coupler, and mixers, as well as the size, weight, and DC power required for the hardware associated with a single correlation. RF digital correlators require a significant amount of hardware and/or DC power to operate on bandwidths greater than 150 MHz, and the maximum bandwidth that can be quantized and sampled by the known processing circuitry, such as analog-to-digital converters.
In order to provide the correlation of signals having wide bandwidths, it was necessary in the art to -combine multiple correlation systems, where the input signals were split into separate channels for each separate correlation process. Multiple correlation processes significantly increases the system hardware, and adds additional noise into the processing system that results in a degraded final correlation of the entire signal. Additionally, most electrical signals to be correlated include a sine and cosine component that provided separate in-phase (real) and quadrature phase (imaginary) correlation components. In order to provide correlation for both components, it has been necessary in the art to provide distinct correlation processes to separately correlate the two components to provide both magnitude correlation and phase correlation. The correlation of the two separate signal components also adds significant hardware to the correlation processing circuitry.
Many applications exist in the art that would greatly benefit from the correlation of electrical signals having a wider bandwidths, for example, on the order of 3-10 GHz. These applications would also benefit from a reduction in system hardware to provide a reduction in size and weight requirements. It is therefore an object of the present invention to provide a correlation system operable at relatively wide bandwidths without the need for redundant correlation hardware.
In accordance with the teachings of the present invention, an optically implemented wide bandwidth correlation system is disclosed that employs a multi-mode imaging device and a particular modulation format to provide both in-phase and quadrature phase correlation components in a single correlation process. The correlation system includes an optical source that generates a laser beam that is split into a first beam path and a second beam path. The first split beam and a first electrical signal are applied to a first modulator in the first path, and the second split beam and the second electrical signal are applied to a second modulator in the second path. The modulated beams are then applied to the optical imaging device, such as a multi-mode imager, that causes the beams to interfere with each other within an optical cavity. Four optical outputs are connected to the optical cavity at strategic locations to provide a zero phase output and a xcfx80 phase output that represent the in-phase correlation component, and a xcfx80/2 quadrature phase output and a 3xcfx80/2 quadrature phase output that represent the quadrature phase correlation component. A photodetector detects the output signals form the imaging device to provide electrical signals indicative of the optical outputs. A first differential amplifier receives the electrical signals for the in-phase component and a second differential amplifier receives the electrical signals for the quadrature phase component. The differential amplifier outputs are applied to separate integrators to sum the signals for the correlation process.
In one embodiment, the modulators employ a Mach-Zehnder interferometer modulator, to provide one of a single sideband suppressed carrier modulation, a double sideband suppressed carrier modulation, a single sideband with carrier modulation, or a double sideband with carrier modulation depending on the particular design. To simultaneously provide both the in-phase and quadrature-phase correlation components by the single imager, the single sideband suppressed carrier modulation format is employed.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.